Global July temperatures were the fourth-warmest on record. (Graphic courtesy NASA)Updating their official stance on the relationship between climate change and weather events, the American Meteorological Society (AMS), has now stated plainly <http://www.ametsoc.org/policy/2012climatechange.html> that the warming world is a fact beyond reproach and that human activity—despite some natural variability—is the main force behind such changes.

"Warming of the climate system now is unequivocal," the group said in a statement <http://www.ametsoc.org/policy/2012climatechange.html> on Monday. "Observations show increases in globally averaged air and ocean temperatures, as well as widespread melting of snow and ice and rising globally averaged sea level."

The dominant cause of the warming since the 1950s is human activities, they said, adding that their scientific finding is based on "a large and persuasive body of research."

"Due to natural variability," the statement noted, "not every year is warmer than the preceding year globally. Nevertheless, all of the 10 warmest years in the global temperature records up to 2011 have occurred since 1997, with 2005 and 2010 being the warmest two years in more than a century of global records."

"This statement is the result of hundreds of hours of work by many AMS members over the past year," comments AMS Executive Director Keith Seitter. "It was a careful and thorough process with many stages of review, and one that included the opportunity for input from any AMS member before the draft was finalized."

Turning to the causes of such drastic changes, the AMS concluded that although the climate is always changing in some respects, the melting of the polar ice sheets, extreme flooding, intense droughts and the retreat of most of the world's glaciers "are beyond what can be explained by the natural variability of the climate."

They continue: "It is clear from extensive scientific evidence that the dominant cause of the rapid change in climate of the past half century is human-induced increases in the amount of atmospheric greenhouse gases, including carbon dioxide (CO2), chlorofluorocarbons, methane, and nitrous oxide. The most important of these over the long term is CO2, whose concentration in the atmosphere is rising principally as a result of fossil-fuel combustion and deforestation."

The AMS' new language follows pressure from some environmentalists and scientists who have criticized the meteorologist professionals for being ambiguous about the growing scientific evidence about the relationship between weather events and global climate change.